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This invention relates to the transfer of electrical power via induction and in particular to an inductive power transfer and distribution system using a coaxial transformer for electric vehicles.
Electrically powered vehicles are useful in manufacturing and warehouse environments for transporting materials in automated material handling systems. Electrically powered vehicles are desirable in these environments due to their clean operation and low noise. In particular, electrically powered vehicles in a material handling system are particularly useful in semiconductor manufacturing facilities. These systems are able to transport semiconductor material to be processed throughout the facility in a coordinated manner to increase the efficiency of the manufacturing process. Material handling systems often have a fixed dual-rail or monorail system on which an electric vehicle travels to and from assigned destinations. This allows for the precise control of the movement of material along a predetermined path within the facility.
Electric vehicles, however, require sufficient electrical power to have any meaningful mobility and speed. On-board rechargeable energy storage systems, such as batteries, have a significant mass that must be moved in addition to the mass of the material to be transported and the mass of the electric vehicle itself. The mass of the batteries decreases the range and speed of the electric vehicle and as such reduces the time between battery rechargings. Typically, the time between battery rechargings depends upon the mass of the material to be moved and the frequency of use. Accordingly, some form of electrical coupling to a power source or power distribution system is required to recharge these systems without requiring the electric vehicles to be taken out of service. Physical contact between a moving electric vehicle and a power distribution system is often unreliable and has other problems associated with it as well. Brush contact and pantograph are typical prior art methods of coupling a moving electric vehicle to a power distribution system. These prior art methods, however, create a risk of sparking in potentially volatile atmospheres, introducing dirt and grease into an otherwise clean environment, or increasing the risk of a mechanical failure that may disable all or part of the material handling system.
Non-contact forms of power transfer are often used in electric vehicle transportation and material handling systems to provide the primary power to the electric vehicle or to recharge one or more on-board rechargeable power sources. Typically, prior art systems use a form of inductive power transfer to provide power to the electric vehicle in a non-contact manner. These systems typically are configured as a primary coil and a secondary coil in a primary-void-secondary configuration. The secondary coil, which is attached to the electric vehicle, is typically placed on the center post of an E-shaped structure. The primary coil is typically formed by one or more pair of parallel wires that form first and second sides thereof. The current flow in the first and second sides of the primary coil is in opposite directions to generate a magnetic field that is coaxial with the longitudinal axis of the coil. During operation, the center post of the E-shaped structure, on which the secondary coil is disposed, passes between the pair of parallel wires that form the first and second sides of the primary coil. An alternating current is imposed upon the primary coil generating a varying magnetic field that is coupled to the secondary coil, inducing a voltage therein.
This primary-void-secondary configuration, however, allows leakage of the magnetic field and radiates electromagnetic interference (EMI). Leakage of the magnetic field can cause heating of adjacent ferromagnetic structures that can change the physical, electrical, or magnetic properties thereof. EMI can interfere with circuits and data transmission resulting in the loss of time and efficiency. In addition, the use of a long primary of parallel wires results in a very high inductance. In order to achieve useful power levels high voltages must be used. The use of high voltages, often in the hundred of volts range, increases the risk of accidents and increases the cost of the system due to the components needed to manage the higher voltages.
Therefore, it would be advantageous to have an inductive power transfer system that contained the magnetic field and reduced EMI and required lower voltages for operation.
A coaxial inductive power transfer and distribution apparatus is disclosed that includes a primary conductor that is stationary and a mobile secondary coil magnetically coupled to the stationary primary conductor to provide an inductive power transfer therebetween. The primary and secondary coils are disposed within a return conductor that is stationary and acts as a return path for current flowing in the primary center conductor. The primary conductor is mechanically connected to the interior surface of the return conductor to ensure the position stability of the primary conductor with respect to the secondary coil. The return conductor also includes an air gap in which a support member for the mobile secondary coil and structure is disposed within. The mobile secondary coil and structure includes a toroidal core composed of a high permeability material that is coaxially disposed about, and spaced apart from, the primary conductor. A multi-turn coil is radially disposed about the high permeability toroidal core such that magnetic flux produced by a current flowing in the center conductor is coupled to the coil. The secondary structure includes a support element coaxially disposed about the high permeability toroidal core and the multi-turn coil. The support member extends through the air gap in the return conductor to the exterior of the return conductor, where it may be coupled to an electric vehicle. The low inductance of the primary conductor allows for low voltages to be used to power the vehicle electrically coupled to the mobile secondary coil and mechanically coupled to the mobile secondary structure.
A power distribution system is also disclosed that includes a voltage source providing a switched voltage signal having a predetermined frequency and amplitude to a primary coil of a power transformer. The secondary voltage of the power transformer is provided to a resonant circuit that includes a capacitor in parallel with a plurality of transformer primary windings that are connected in series with one another. Each of the plurality of transformer primary windings is magnetically coupled to a corresponding secondary winding. The turns ratio of the primary to secondary windings is such that a predetermined voltage and current are provided in each of the plurality of transformer secondary windings. Each of the plurality of transformer secondary windings is connected to a corresponding center conductor for coupling to a secondary coil to provide power thereto.
Other forms, features and aspects of the above-described methods and system are described in the detailed description that follows.